This application relates to displacement or strain indicators, and more particularly to fluid based opti-mechanical indicators capable of indicating very small displacements or movements of one member relative to another member via a change in intensity or color of an indicator.
There have been a number of attempts to create a low cost, easy to use displacement indicator. For example, U.S. Pat. No. 3,602,186 to Popenoe, which is incorporated herein by reference in its entirety, introduced the concept of using a preferably dark fluid within a mechanical device to create a significant color change when there was relative motion between two portions of the device. In two subsequent patents, U.S. Pat. No. 3,799,108 to Mosow and U.S. Pat. No. 3,850,133 to Johnson, difficulty of manufacture was stated as motivation for the sub-assembly module designs that were described. Another advance in manufacturability is described in U.S. Pat. No. 3,987,699 to Popenoe, which is hereby incorporated by reference in its entirety. The '699 patent described a self-contained flexible container encapsulating the indicator fluid, which is known as a microindicator.
Further patents describe additional developments in microindicators, including U.S. Pat. Nos. 3,987,668, 4,904,132, 5,189,979, and 7,958,614, each to Popenoe, and each of which being incorporated herein by reference in their entirety. One form of displacement microindicator technology was configured for use as a tension indicating fastener. A very common problem when installing a bolt is controlling the amount of “preload” or tension on the fastener. Since it has been historically difficult to measure the tension directly, industry has relied on the use of torque wrenches as a means to control tension. Unfortunately, due to the inherent variability of the torque-tension relationship, any method using measurement of torque to control tension has resulted in intrinsically inaccurate tension results. These inaccuracies and problems remain in industry today.
Manufacturability of the indicator described in the '186 patent has been difficult to improve, and there have been ongoing efforts to make the indicators more efficiently. Two subsequent patents, U.S. Pat. No. 3,799,108 to Mosow and U.S. Pat. No. 3,850,133 to Johnson each describes a “module” design that was able to be manufactured in larger numbers, yet, performance of these designs were unacceptable due to a tendency of the designs to leak fluid, especially over time, resulting in non-operable products. The displacement microindicator technology described by Popenoe in the '699 patent, however, was able to improve upon the leaking design and provide a successful and usable displacement indicator product. Although the microindicator described in the '699 patent improved manufacturability, the microindicator technology utilized polymer based materials that limit application in temperature environments greater than 75° C.